This invention relates generally to radio frequency circuits and more particularly to frequency conversion circuits.
As is known in the art, frequency conversion circuits provide an output signal in response to an input signal having a frequency which is higher or lower than the frequency of the input signal. One type of frequency conversion circuit is the so-called down converter or a mixer. Radio frequency mixer circuits are widely employed in superheterodyne receivers which include in addition to the mixer an intermediate frequency (IF) amplifier tuned to a predetermined frequency provided from the mixer and a fixed frequency detector which is fed the amplified signal from the IF amplifier. Generally, a received input signal and a local oscillator (LO) signal are fed to the mixer circuit to provide an output signal having a pair of frequency components equal to the sum and difference of the frequencies of the input signal and the local oscillator signal. Typically, the sum frequency component is filtered from the signal and the signal having the difference frequency component is fed to the IF amplifier.
One type of mixer is the so-called single-ended mixer. The single-ended mixer generally includes a nonlinear device such as a transistor or a diode which is fed by the input signal and the local oscillator signal. In response an output signal is provided having the sum and difference pair of frequencies (i.e., .omega..sub.rf .+-..omega..sub.LO).
One problem with this type of mixer, however, is that the output signal generally includes undesirable frequency components such as the input signal frequency .omega..sub.rf, the local oscillator signal frequency .omega..sub.LO, harmonics (n.omega..sub.rf, m.omega..sub.LO) of the original input signals, intermodulation products of the harmonics (m .omega..sub.LO .+-.n.omega..sub.rf), and a DC output level. It is generally required to suppress the undesired frequency components, since their presence may cause frequency ambiguity in the receiver. A second problem with single-ended mixers is that the local oscillator signal terminal and the radio frequency input signal terminal generally are not isolated. Thus, a portion of the local oscillator signal may feed through to the radio frequency terminal causing radiation of the local oscillator signal and additional interference problems.
A further problem with the single-ended mixer is the so-called image response of the mixer. The desired IF frequency .omega..sub.IF can be produced by an r.f. signal having a frequency above or below the frequency of the local oscillator (.omega..sub.rf =.omega..sub.LO .+-..omega..sub.IF). If one of the IF frequencies is the desired frequency, then the other frequency is termed the image frequency. If one input signal produces the desired IF signal, then the other input signal produces a signal referred to as the image signal. In many applications, it is necessary to either distinguish between the desired and image signals or eliminate the image signal. With a single-ended mixer, there is generally no way to distinguish between the desired signal and the image signal; therefore, filtering is employed to eliminate the image signal. Fixed frequency filtering, however, is possible only when there is no overlap in the bandwidth of the desired signal and the image signal and tunable frequency filtering are generally difficult to fabricate over wide bandwidths, particularly, as integrated circuits. Therefore, for broadband applications, filtering is generally not successfully employed with single-ended mixers.
A third problem with the single-ended mixer is that the mixer not only responds to a signal at the image frequency, it may produce a signal at the image frequency. The mixer generally produces such a signal in one of two ways: First, an r.f. signal having a frequency higher or above the local oscillator frequency (given by .omega..sub.rf =.omega..sub.LO +.omega..sub.IF) if mixed with the second harmonic of the local oscillator signal 2 .sub.LO will produce a signal at the image frequency (.omega..sub.IM) (given by .omega..sub.IM =2.omega..sub.LO -.omega..sub.rf).
Alternatively, an impedance mismatch at the IF output terminal will produce a reflected signal which will propagate back towards the mixer. This reflected signal mixes with the local oscillator signal to produce a signal either above or below the local oscillator signal (.omega..sub.LO .+-..omega..sub.IF), one of which is at the image frequency. Nevertheless, in either case, the signal produced at the image frequency increases the conversion loss of the mixer, or in other words, reduces the efficiency by which the mixer converts the input signal to the desired IF signal.
A second type of mixer is the so-called balanced mixer. A balanced mixer generally includes a pair of single-ended mixers and a 3 db hybrid coupler. The inputs of the coupler are fed the input signal and local oscillator signal and outputs of the hybrid are coupled to the input of each single-ended mixer. The outputs of the mixer elements are combined in a common junction to provide the output IF signal. The balanced mixer generally has a relatively high isolation between input and local oscillator signals if the outputs of the hybrid are properly terminated thereby reducing re-radiation of the LO signal. The hybrid provides a predetermined phase shift between the input signal and local oscillator signal generally equal to 90.degree. or 180.degree.. Balanced mixers having a 90.degree. phase shift between the local oscillator signal and the input signal are generally used to suppress harmonics and intermodulation products for both the input signal and the local oscillator signal. Balanced mixers having a 180.degree. phase shift between the signals are generally used to suppress the even harmonics of the local oscillator frequency signal. One problem with either of the balanced mixers is that while the above-recited balanced mixer configurations suppress some of the unwanted frequency components in the output, certain others of those frequency components are not suppressed. A second problem is that the balanced mixer includes a hybrid coupler, a generally difficult circuit to fabricate, particularly as an integrated circuit. Furthermore, the balanced mixer may produce energy at the image frequency. Also, the balanced mixer cannot differentiate between input signals having frequencies either above or below that of the local oscillator signal. Further still, since the balanced mixer requires the use of a hybrid coupler, the bandwidth of the balanced mixer is generally limited due to the limited bandwidth of the hybrid coupler.
A third type of mixer is the so-called double balanced mixer. The double balanced mixer generally includes two balanced mixers, each fed by the local oscillator signal and one of a pair of input signals having a 180.degree. differential phase shift provided by a hybrid coupler or balun. The outputs of each balanced mixer are combined together by an IF hybrid coupler. One problem with double balanced mixers is that the couplers or baluns are generally difficult to fabricate as monolithic integrated circuits. Accordingly, the double balanced mixer is not easily fabricated as a monolithic integrated circuit. A second problem with the double balanced mixer is a relatively narrow frequency bandwidth of operation of the mixer due to the presence of the couplers and baluns. Further, while certain configurations of double balanced mixers may distinguish between signals having a frequency either higher or lower than that of the local oscillator, those configurations of the double balanced mixers may still produce a signal at the image frequency.
As is also known in the art, a second type of frequency conversion circuit, an up converter, provides an output signal having a frequency which is higher than the frequency of the input signal. As mentioned above, a nonlinear device when fed input and local oscillator signals, provides an output signal having frequency components equal to the sum and difference between the frequencies of said signals. Therefore, by filtering the difference frequency component, the remaining sum frequency component provides a signal having a frequency which is higher than the frequency of the input signal.